Probably more like 400 bytes, which doesn't sound like that much but most of the other categories add around just 100 bytes each.

What I'm worried about is that it would have to be added to the common key-detection code, which would slow down conversion for all modes slightly. (All current conversion modes, except for Temperature, don't affect the main code at all.)

So in the programming you just store then like the ratio (exept for temp) and then put them into the same function?

Yep, that's why it's so small, and that's why Temperature and Base conversion are both heavier than the others—they don't align to ratios very nicely.